Our Moon
Goals of this Lecture Understand the Lunar Phases Introduce tides and tidal forces Understand Lunar Eclipses Understand Solar Eclipses
Assigned Reading and Announcements Chapter 3, all of it Homework #1 will be posted today Due in class on Thursday 18
Lunar Phases
Earth & Moon Like planets, the moon does not emit its own right. The Moon is ~1/4 of the Earth in diameter and ~1 percent in mass. The rotation of the Moon and its revolution around the Earth are synchronized and take 27 days,7 hours, and 43 minutes. The length of the Lunar orbit around Earth (Sidereal Period) is 27.32 days. The length of the Lunar Phase (Synodic Period) is 29.53 days. We only see one side of the moon.
The Phases of the Moon (1) The Moon orbits Earth in a sidereal period of 27.32 days. Earth 27.32 days Moon Fixed direction in space
The Phases of the Moon (2) Fixed direction in space 29.53 days Moon Earth Earth orbits around Sun => Direction toward Sun changes! The Moon’s synodic period (to reach the same position relative to the sun, i.e., the same lunar phase) is 29.53 days (~ 1 month).
One rotation in 24 hours One orbit in 27.32 days
Lunar Phases sunlight As seen from Amherst
Survey Question Lunar Phases sunlight As seen from Amherst
A third quarter moon is rising. What time is it? 1) sunset 2) sunrise Survey Question A third quarter moon is rising. What time is it? 1) sunset 2) sunrise 3) midnight 4) noon
Earth Rising on the Moon Apollo 11
Survey Question In this picture of the Earth rising, what is the predominant direction of the sun in relation to the observer? 1) above the observer 2) below the observer 3) behind the observer 4) behind the Earth
How long does it take for the moon to rotate once? One day About 27 days Half a year One year
How long does it take for the moon to rotate once? One day About 27 days Half a year One year
Lunar and Solar Eclipses Lunar, from “Luna” Solar, from “Sol”
Lunar Eclipse Whose shadow is this? Why is the shadow curved?
Reminder: angular (apparent) diameter Angular diam / 206,265” = linear diam./distance NOTE: linear diam. and distance MUST be in the same units It so happens that both the Moon and the Sun have angular diameter of about 0.5 degree, the exact value at a given time depending on the exact distance.
Shadows
A Total Lunar Eclipse (1) Motion of the moon against the background of stars Motion of the moon against the horizon
A Total Lunar Eclipse (2) A total lunar eclipse can last up to 1 hour and 40 min. During a total eclipse, the moon has a faint, red glow, reflecting sun light scattered in Earth’s atmosphere.
Partial lunar eclipse
During a lunar eclipse, what would you see from the moon?
Geometry of Solar Eclipses Umbra (shadow): sun is completely blocked out. Penumbra (almost shadow): sun is partially blocked out.
Earth and Moon’s Orbits Are Slightly Elliptical Apogee = position furthest away from Earth Earth Perihelion = position closest to the sun Moon Perigee = position closest to Earth Sun Aphelion = position furthest away from the sun (Eccentricities greatly exaggerated!)
Annular Solar Eclipses The angular sizes of the moon and the sun vary, depending on their distance from Earth. When Earth is near perihelion and the moon is near apogee, we see an annular solar eclipse. Perigee Apogee Perihelion Aphelion
Annular eclipse
Credit: Mir Space Station Crew Looking Back on an Eclipsed Earth; 1999 August 11 Credit: Mir Space Station Crew
Total Eclipse of June 21st 2001 The umbra is never more than 270 km in diameter and sweeps across Earth's surface at 1600 km/hr (1000 mph)!
Survey Question The phase of the moon at a solar eclipse is? a) new b) 1st quarter c) full d) it depends on the year
How come there isn't an eclipse every full moon and new moon? The main reason is that the moon’s orbit is tilted from the earth’s orbit. A total eclipse from a given point on the surface of the Earth is not a common occurrence. For example, it will be two decades before the next total solar eclipse visible in North America occurs.
Draw a diagram showing the related objects (Earth, the moon, or Sun) during a) a partial solar eclipse, b) an annular eclipse, c) a total lunar eclipse, d) a partial lunar eclipse.
Effects of Moon on Earth: Tides To sun
Spring Tides Occur at every new and full moon
Neap tides Occur at every first- and third-quarter moon
Earth's rotation slows down by 0.0023 s/100 years. Friction drags the tidal bulges eastward out of the direct earth-moon line Earth's rotation slows down by 0.0023 s/100 years. Only 900 million years ago, Earth' day was 18 hrs long. The moon's orbit is growing larger by about 4 cm/yr.
We see only one side of the moon, because the tidal friction has slowed the rotation of the moon so that its rotation and revolution are synchronized
The Tidally-Locked Orbit of the Moon The Earth exerts tidal forces on the moon’s rocky interior. It is rotating with the same period around its axis as it is orbiting Earth (tidally locked). We always see the same side of the moon facing Earth.
Solar day and Sidereal day The apparent solar day = the time from high noon to high noon. The average length of the apparent solar day = 24 hours Sidereal day = the time from a star to the same position. Because the sun moves in respect to stars, the sidereal day is different from the solar day.
The Length of the Solar Day Earth revolves about 360o/365 ~ 1o around Sun Earth rotates at a rate of 360o/(24x60)=0.25o/minute So need an extra 1o/0.25o =4 minutes to complete a solar day